The invention relates to the control of laser machining, particularly of semiconductor material such as silicon wafers, Gallium Arsenide, Silicon Germanium, Indium Phosphide and others.
Machining a semiconductor wafer is conventionally achieved by dicing the wafer with a saw. For example, EP0202630 describes the use of a dicing saw for machining streets in silicon wafers. Mechanical machining has disadvantages such as low yield, chipping and cracks. Thin wafers cannot be machined and specialised applications such as machining curved structures, internal through holes, etc., are not possible.
It is also known to use laser beams for machining semiconductor substrates. U.S. Pat. No. 5,214,261 describes a method where deep ultraviolet excimer laser beams are used to dice a semiconductor substrate. Excimer lasers, however, do not cut with sufficient speed for many applications.
Other lasers such as Nd:YAG (1064 nm) and CO2 laser have also been used in semiconductor substrate micro-machining. These lasers generate debris and large heat affected zones. U.S. Pat. No. 4,224,101 describes the use of a Nd:YAG laser to form grooves in a semiconductor. This step is followed by a cleave and break step along the grooves. A further step requiring chemical etching was used to remove the debris and hot particles that land on the wafer and fuse to the surface during machining.
These lasers do not appear to have been applied successfully to precision applications. The reason for this is that the quality of the edge formed is not acceptable. Also, heat is generated at the cut front, this may result in damage to the electrical function for which the component was manufactured. Heating of the substrate material induces thermal stress in the wafer which can cause microcracks, having a deleterious effect on lifetime and function.
U.S. Pat. No. 5,916,460 describes the use of a defocused beam and a high-pressure flow of assist gas to suppress the generation of microcracks. A defocused beam incident on the surface of the wafer generates a crack, which propagates along the dicing direction. Controlling such a process is difficult.
This invention is therefore directed towards providing for improved machining of semiconductor material. The improvements specifically, are that the process throughput and quality are sufficient to allow low cost manufacturing of components as well as enabling the manufacture of precision micro-machined structures such as micro-fluidic devices.
According to the invention, there is provided method of machining a semiconductor material using a laser beam in which a formation is machined in the material to a width S using a laser beam of intensity IB, and in which the beam is controlled to machine the material with a kerf K,
characterised in that, the beam is controlled to scan n times, n being nxe2x89xa71 and, where n greater than 1, each subsequent scan is laterally offset and parallel to a preceding scan, and n is xe2x89xa7S/K.
In one embodiment, the value for IB is chosen to lie in a range of values of IB for which material removal rate increases with increasing IB 
In a further embodiment, IB is in a range for which material removal rate increases at a rate of at least 30% with increasing intensity.
In one embodiment, the lateral offset between scans is in the range from one micron to the kerf K.
In a further embodiment, the lateral offset between scans is selected by varying the lateral offset in steps from one micron to the kerf until the net machining speed is optimised.
In one embodiment, machining is achieved by repeating scans with nxe2x89xa71 in each of a number of steps (z) so that material is removed in a sequence of steps from the surface downwards.
In one embodiment, the beam dimensions at focus are controlled so that the beam intensity, IB, results in minimisation of the total number of scans required to define the required formation.
In one embodiment, the laser beam is pulsed, and the pulse repetition frequency and scan speed are chosen to provide a pulse overlap in the range of 30% to 98%.
In another embodiment, the laser beam is pulsed, and the pulse overlap is selected in the region of 30% to 85% to control and refine the texture and roughness of the walls of a machined channel or the walls and bottom of a machined trough and to clean residual debris.
In one embodiment, the channel width (S) is chosen so that the net machining speed is fastest when compared to the machining speed for larger or smaller channel widths as machined under optimal values for the number of parallel laser lines for that particular channel width.
In one embodiment, the laser beam wavelength is in the range of 350 nm to 550 nm, the repetition frequency is greater than 5 kHz, and the average laser beam power is greater than 3 W.
In another embodiment, the laser beam wavelength is in the range of 250 to 300 nm, the repetition frequency is greater than 1 kHz, and the average power output is greater than 1 W.
In one embodiment, scan velocity, laser power, and pulse overlap are chosen to control depth of material removal in any one scan.
In another embodiment the method comprises the further step of, after machining, performing a final laser scan in which:
the beam diameter is greater than the width S, and the beam intensity is below a machining intensity threshold;
whereby a machined formation is cleaned.
In one embodiment, the method is performed to machine through channels to singulate die.
In another embodiment, the semiconductor material is a substrate for a micro-fluidic device.
In a further embodiment, a trench is formed in a surface of the substrate, the trench being suitable to act as a fluid delivery channel of a micro-fluidic device.
In one embodiment, the number of scans and lateral offsets of the scans is varied so that a tapered structure is formed in the material.
In another embodiment, a tapered structure is formed in a circular or elongated aperture.
In a further embodiment, a plurality of tapered structures are machined to form wells, funnels and through hole channels of the micro-fluidic device.
In one embodiment, the material is machined from a top side, and subsequently from a bottom side, the formations from the sides joining to form a single through formation.
In one embodiment, a top side camera and a bottom-side camera are aligned and calibrated such that a transformation mapping coordinates of the top camera to coordinates of the bottom camera is known, and the top side and bottomside material coordinates are registered with respect to each other for registration of machining on both sides.
In one embodiment, machining of the material from both sides enables the formation of curved and tapered elongate and circular wall structures.
In another embodiment, a fume extraction head is used for extraction of fumes and solid debris from above and below the material, and wherein assist gas is directed at the material to control the deposition of debris and assist the machining process.
According to another aspect, the invention provides a laser machining apparatus comprising a laser source, means for directing a laser beam from the source at a semiconductor material to machine with a kerf K to a width S, and a controller for controlling parameters of the laser beam,
characterised in that,
the controller comprises means for directing the laser beam in a plurality of n parallel passes, said passes being laterally offset, and wherein n is greater than or equal to S/K.
In one embodiment, the controller comprises means for controlling laser beam intensity (IB) so that it lies in a range of values of intensity for which material removal rate increases with increasing intensity.
In one embodiment, the laser machining apparatus further comprises a fume extraction system having suction inlets above and below the material support means.
In one embodiment, the laser machining apparatus further comprises a gas blowing system comprising nozzles for directing an assist gas over the material being machined.
In another embodiment, the laser machining apparatus further incorporates a vision system which consists of top and bottom camera systems in registration with each other, and a controller comprising means for using images from the cameras to ensure registration of the material after flipping.